Deuterium Isotope Effects in the Solvolysis of Benzal Chlorides. 5. Salt Effects and Solvent Isotope Effects

Article abstract of DOI:10.1021/ja00398a018

Rates and solvent isotope effects have been determined for the solvolysis of p-methoxybenzal chloride (1) and diphenyldichloromethane (2) in 75percent (v/v) dioxane-25percent H2O and 85percent (v/v) dioxane-15percent H2O in the presence of various salts.Both 1 and 2 have solvolysis rates which follow the Winstein equation for the "special" salt effect, i.e., k_obsd = k⁰_ext(1 + b_S).For both 1 and 2, k⁰_ext/k⁰_obsd is identical for the two different salts LiClO4 and Bu4NClO4 in 85percent dioxane.For dissociation of intimate ion pair rate limiting in 85percent dioxane for 1 and 2, the solvent isotope effect is ca. 1.3.Rate-limiting capture of ion-pair intermediates in 75percent and 85percent dioxane furnish solvent isotope effects of ca. 1.5.It is suggested that these differences are due predominantly to the presence or absence of some covalent bonding to water in these transition states.